1. Technical Field
The present invention relates to a semiconductor light-receiving element, particularly to a semiconductor light-receiving element fabricated by an impurity diffusion method.
2. Prior Art
A photodiode is generally used as one of photodetectors. A photodiode is fabricated by forming an N-type region in a P-type region of semiconductor and vice versa to form a PN junction. When light impinges upon the PN junction, free carriers (electron-hole pairs) are generated in the PN junction. The carrier are drifted to the P-type or N-type region through the electric field induced by space charge in the PN junction or the electric field intentionally applied between the P-type and N-type region. By the drift of the carriers, a current (or a voltage) is generated between P-type and N-type regions, so that the presence or strength of light may be sensed by monitoring the current. Furthermore, a PIN photodiode including a region of low impurity concentration formed between P-type and N-type regions has been fabricated for the purpose of a high sensitivity and high speed.
Compound semiconductors such as silicon, gallium arsenide, and the like have been broadly used as materials for a photodiode having a sensitive peak to visible light. Alternatively, Ge and InGaAs are used for sensing near infrared ray. A photodiode have generally the structure in which an impurity is diffused into one of the materials described above or into a part of the material epitaxialy grown said one of the material to form an island region having a conductivity type different from that of a substrate. Electrodes are formed on the top of impurity diffused region and the bottom of the substrate.
FIG. 1 shows a partially cutaway perspective view of a conventional photodiode, and FIG. 2 illustrates a plan view thereof. An impurity diffused region (or an active region) 10 formed in a substrate 8 is shown by a hatching area. An annular electrode 12 is formed on the top of the diffused region 10, and a flat electrode 6 is formed on the bottom of the substrate 8.
In this type of photodiode, if the active region 10 is small, then the light impinged upon the photodiode is spread outside the active region 10. As a result, it becomes difficult that all of the incident light is absorbed by the active region 10. The spread of the incident light outside the active region is considered due to the following reasons, for example;
(1) the convergence of light is insufficient,
(2) a part of light impinges upon the area outside the active region due to the shift of optical axis, and
(3) the light passing through the active region is reflected or scattered on the electrode provided on the bottom of the substrate and is absorbed in the area outside the active region.
The carriers generated in the active region are accelerated by reverse vias applied to the PN junction to cause a drift current. On the other hand, the carriers generated in the area outside the active region cause a diffusion current. The diffusion current has an influence on the output of the photodiode. That is, when a part of light is absorbed in the area outside the active region, the output of the photodiode with respect to the light input is decreased. Also, the carriers generated in the active region and then overflowed into the area outside the active region, and the carriers generated in the area outside the active region are diffused and recombinated in delayed. This causes the phenomenon such that the following-up to pulsed light is delayed, and then the photoelectric conversion for a high-speed digital signal may not be effectively performed. The phenomenon are generally referred to as xe2x80x9ca slow tail phenomenonxe2x80x9d in the output in a photodiode, resulting in bit errors.
In order to suppress the occurrence of such a slow tail phenomenon, the following methods are used, i.e. (1) a floating guard ring method, and (2) a method for shielding light by using a metal film and the like.
Referring to FIG. 3, according to the floating guard ring method, a floating guard ring 14 is provided in a region surrounding the active region 10. The floating guard ring 14 is formed by diffusing an impurity in a substrate in the same manner as the formation of the active region 10. Therefore, the floating guard ring also includes a PN junction. The internal electric field caused in the PN junction promotes the recombination of generated carriers to suppress the slow tail phenomenon.
Referring to FIG. 4, according to the latter method for shielding light, the area outside the active region 10 is covered by a light shielding film 16 in order to prevent light from impinging upon the area.
In the floating guard ring method, all of the carriers generated in the area outside the active region 10 and flew into the area are not necessarily recombined, because not only drift but also diffusion are operated in the floating guard ring.
The electric field between the active region 10 and the floating guard ring 14 is zero. Therefore, when light impinges upon between the active region 10 and the floating guard ring 14, the carriers generated therebetween migrate to the active region 10 or the floating guard ring 14 by diffusion. The carriers reached to the floating guard ring are not necessarily combinated therein as described above. Accordingly, the float guard ring method may not suppress enough the slow tail phenomenon.
The method for shielding light by using a metal film and the like is effective to the insufficiency of convergence of light and the optical misalignment, but is not effective to the reflection and scattering of light passing through the active region on the bottom electrode, and the overflow of carriers from the active region to the area outside the active region. In this manner, the light shielding film 16 may prevent light from impinging, but have no effect on the prevention for mutual diffusion of carriers between the active region and the area outside the active region.
When an optical fiber is coupled to the conventional photodiode, the optical fiber is aligned to the photodiode while outputting light from the optical fiber and monitoring the output of the photodiode. There are two methods for such alignment, i.e. one is a DC alignment method in which an optical fiber is aligned to a photodiode so that the output of the photodiode becomes maximum while outputting continuous light (DC light) the strength thereof does not varied from the optical fiber, and the other is an AC alignment method using AC light the strength thereof is varied periodically. The optimum alignment position obtained by the DC alignment method and that by the AC alignment method are sometimes different.
The carriers generated in the area outside the active region are diffused into the active region to contribute the output of a photodiode in the case of DC light, while in the case of AC light, the diffusion of carriers may not follow with the variation of the strength of AC light when the frequency thereof is high. Therefore, the output of the photodiode in the case of AC light is different from that in the case of DC light. Accordingly, the results of alignment are different in the DC alignment and AC alignment methods depending on the state of carriers generated in the area outside the active region.
An object of the present invention is to provide a semiconductor light-receiving element fabricated by using an impurity diffusion, in which a slow tail phenomenon caused in the processing of a digital signal may be suppressed.
Another object of the present invention is to provide a semiconductor light-receiving element in which when an optical fiber is coupled thereto, the optimum alignment position has no difference between a DC alignment and AC alignment.
The present invention provides a semiconductor light-receiving element, comprising:
a substrate including a first impurity diffused region,
a first electrode provided on the bottom of the substrate,
a second electrode provided on the first impurity diffused region,
a second impurity diffused region provided so as to surround the first impurity diffused region with leaving a certain space therebetween, and
a third electrode provided on the second impurity diffused region,
wherein a reverse vias is applied to a PN junction formed by the substrate and the second impurity diffused region.
The present invention provides a method for coupling an optical fiber with the semiconductor light-receiving element according to claim 1 or 2, comprising the steps of:
shifting an end of the optical fiber with respect to the first impurity diffused region of the semiconductor light-receiving element while outputting light from the optical fiber,
monitoring the outputs of the first and second impurity diffused regions, and
determining optimum alignment position of the optical fiber with respect to the semiconductor light-receiving element, when the output of the first impurity diffused region is largest and the output of the second impurity diffused region is smallest.